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# Determining the Concentration (Molarity) of Blue … - convert concentration to molarity

Determining the Concentration (Molarity) of Blue Dye in Gatorade

Introduction:

We have seen how colors appear to our eyes as a result of white light falling on those colors and the colored object absorbing all of the colors from the white light EXCEPT those of the color. For example, a blue dye appears blue because white light falls upon the blue dye and the dye absorbs ALL colors except blue. Blue is then reflected into our eyes.

A Spectrophotometer is a device that measures the amount of light of a particular wavelength (color) that comes through a solution. Light of all wavelengths is shined on the sample, and the spectrophotometer is set at a particular wavelength to detect if that wavelength passes through the sample.

We are going to make several solutions of blue dye (which has a wavelength of approximately 480-430 nanometers) and pass a white light through it to see how much ORANGE wavelength (yes, ORANGE) gets through at 630 nm.

Why do you think we choose the range of 630 nm to detect when blue dye is actually in the range of 480-430?_______________________________________

Solution Preparation:

The data table below tells you how to prepare your eight-samples from the stock solution of 1 Molar (1 mole/liter) Blue Dye.

|Solution # |Dilution Ratio mL |Molar Concentration |Percent Transmittance |Percent Transmittance in|1/decimal form of |
| |Stock/mL H2O |(Moles/Liter) |(%T) |Decimal form |percent transmittance |
| | | | | |(1/T) |
|1 |0 mL/10 mL |0 M |  |  |  |
|2 |1 mL/ 9 mL |  |  |  |  |
|3 |2 mL/ 8 mL |  |  |  |  |
|4 |3 mL/ 7 mL |  |  |  |  |
|5 |4 mL/ 6 mL |  |  |  |  |
|6 |6 mL/ 4 mL |  |  |  |  |
|7 |8 mL/ 2 mL |  |  |  |  |
|8 |10 mL/ 0 mL |1 M |  |  |  |

Your first solution sample will be pure water. Your second will be 1 mL of stock solution and 9 mL of pure water….etc.

How to determine the Molar Concentration (Moles/Liter):

You will notice that the molar concentration is filled in for you for solutions #1 and #8. Obviously, Solution #1 is a pure water that has no molarity of blue dye
(0 Moles/Liter). Solution #8 is pure Blue Dye and so it has a concentration of 1 M.

To determine the others, you need to use the formula: M1V1 = M2V2
M1 = molarity of stock solution (always 1 M in each solution. That’s the stock molarity)
V1 = Volume amount (mL) of stock solution to be added (varies)
M2 = Resulting molarity (Molar Concentration in Moles/Liter)
V2 = Volume amount (mL) of resulting solution (always 10 mL in each solution)

Obviously, you have three of the four variables available to you and you need to solve for the fourth (Molar Concentration).

Show your work below for each of the Molar Concentrations for solutions 2-7

Solution 2________________________________________________________

Solution 3________________________________________________________

Solution 4________________________________________________________

Solution 5________________________________________________________

Solution 6________________________________________________________

Solution 7________________________________________________________

How to determine the Percent Transmittance:

Once you have your samples made, take them to the spectrophotometer and carefully put each sample into a dry cuvette (clear plastic thing). Record the Percent Transmittance on the data table above.

Convert percent transmittance into a decimal (example: 90% = 0.9). Record on data table above.

Find the reciprocal of the decimal form and record that as 1/T on the data table above.

What are you going to Graph?
Make a graph of 1/T vs. molar concentration

Make a linear fit (If done correctly, this should be a straight line graph)

What does molarity measure the concentration of? Use molarity as a conversion factor in calculations. Another way of expressing concentration is to give the number of moles of solute per unit volume of solution. Of all the quantitative measures of concentration, molarity is the one used most frequently by chemists. Molarity is defined as the number of moles of solute per liter of solution.